The laser instrument incorporates a display of some sort in order to aid users while playing notes. The main function of the display is to allow users to visually see the notes they are playing. This promotes the instrument’s ease of use. If there are eight notes for every octave, the light crystal display (LCD) should display the layout of each type of note that is played. The display provides the instrument with a means to learn the instrument easier and allow larger audiences to adapt to the instrument. While choosing the display, a display with a simple design and ease of use was the preferred choice. This was because the display needed to have a straightforward and simplistic function; therefore, an advanced display would have been too much for this use case.
3.5.1 Research
The project demanded that the device contains a graphical display according to the requirements. There were a multitude of graphical displays that could be
chosen for this project; however, it must uphold the requirements of the project.
Given the requirements, there were a few options to choose from. The major characteristics that should be sought after are cost and size. The project would benefit from a decent size that is easily visible, but also not too big. Of course, the cheaper the display, the better. Some candidates for a viable graphical display are an array of seven-segment displays, electronic ink, and a light crystal display.
3.5.1.1 Seven-Segment Display
The seven-segment display has a quite simple design with typically ten pins controlling each segment of the seven-segment display, a decimal point pin, and two COM pins for common cathode and common anode. A major advantage of the seven-segment display is that the design is simple, easy to operate, and is very low on cost. However, the disadvantage is that there are nine pins that are required to drive the display. If there is more than one singular seven-segment display, the number of pins needed to drive the display increases significantly.
One way to combat this disadvantage is to use auxiliary components such as the shift register integrated circuit. To operate the display, connections need to be made to every pin corresponding to each segment, one for the COM pin, power, and ground. This requires nine connections to drive the display. Adding the shift register would decrease the number of pins required to drive the display by almost half. This allows us to connect the eight display pins for the seven-segment display to the eight output pins of the shift register and only use three pins for the shift register to the processing unit.
3.5.1.2 Electronic Ink
Electronic ink is a very interesting choice for a graphical display. One aspect of the electronic ink display is that the image remains on the display even when the power is cut off. This means that power consumption can be decreased for the system.
This would help relieve the power source slightly and contribute to the requirement R.D.4, in Table 2, which states that the instrument will have an operational lifespan of at least one hour.
Electronic ink comes in a variety of sizes which is great for scalability if the instrument were to change dimensions. The largest complication, though, is that the cost of electronic ink is quite expensive in comparison to other graphical displays. The electronic ink displays on the cheaper end are too small to be able to enhance user experience and the ideal sizes are just a little too costly. One specific part that was found is the ePaper which is one of the decent sized displays with a medium to high cost. The ePaper features a display with a resolution of four hundred by three hundred pixels.
3.5.1.3 Light Crystal Display
Light Crystal Displays, or LCDs, can be a bit too much for this project’s use case.
The LCD has about the same electrical characteristics as the seven-segment display but can provide much more utility. While the seven-segment display is constricted to the seven segments on the display, the LCD provides a larger range of versatility. Since the LCD uses a grid of cells to produce an output, letters are more easily distinguishable and provide the opportunity to create custom characters to display. Although the LCD is a bit more complex than the seven-segment display, however, it still provides an ease of use through simple programming methods.
In a typical 16 by 2 light crystal display, there are two rows of sixteen characters and requires sixteen pins to drive the display. The LCD can be paired with an Inter-Integrated Circuit (I2C) module which helps reduce the number of pins needed to drive the display. Pairing with an I2C module allows the LCD to be driven with four pins just as the seven-segment display with a shift register integrated circuit.
Comparing this with the seven-segment display, only one LCD module would be needed to display everything needed for the instrument. The pins needed to drive the LCD would be provided to the to the I2C module which sends output to the sixteen pins on the LCD.
3.5.2 Technology Comparison
In this specific project, the seven-segment display would have proven to be extraneous due to the number of pins needed to drive multiple displays. The project would need eight seven-segment displays to display eight notes in the octave. This would have required at least twenty-five pins for all eight of the seven-segment display used, assuming the use of shift registers. In comparison, the light crystal display, when paired with the I2C module, reduces the number of pins needed to drive the display to four pins which is only needed for one LCD.
Regarding the cost of each component, the seven-segment display is slightly cheaper than the LCD; however, the project would have still required eight seven-segment displays in order to satisfy the R.D.3 requirement in Table 2. This means that the cost would have been slightly more to implement the seven-segment display in the end. In Table 18, comparisons can be seen between electrical properties, costs, and number of pins needed to drive the displays. The parts being compared in the table are as follows: DV-20200 as the light crystal display, HDSP-515A as the seven-segment display, and the 4.2-inch e-Paper Module from Waveshare as the ePaper.
The ePaper is a component that would work really well with this project if not for the price. Comparing it in the table, the electrical characteristics are phenomenal considering that the ePaper can continue displaying an image even when powered off. The display itself takes up an area of 4.2 inches which could either work well by providing better user experience with a bigger display that is easier to read or
be a potential problem of not being able to fit the dimensions of the instrument frame.
Table 18 LCD versus Seven-Segment Display
LCD Seven-Segment
Display
ePaper Operating Voltage 5 V 1.85 V 3.3 – 5 V
Operating Current 2 mA 20 mA 10 mA
Cost $10.75 $1.53 each $29.99
Pins needed to drive display
4 5 8
The LCD was implemented in this project because the set up required less connections and was cleaner to develop and modify when needed. When looking at electrical properties, the seven-segment display requires more current than the LCD screen and cost less per unit, however, when accounting for eight seven-segment displays, the LCD was the preferred choice. Most importantly, the number of pins is based upon the use of a shift register for the seven-segment display and a I2C interface module for the LCD; therefore, the use of an LCD helps to also cut the cost of multiple shift registers.
3.5.3 Part Selection
The light crystal display was the selected part for the graphic display for several reasons. The LCD is more versatile in terms of displaying characters than the seven-segment display, but about equal with the electronic ink display. The electronic ink would be the most preferred part; however, it is just too costly to consider. The seven-segment display would not be viable for this project because the number of pins needed to drive all the displays needed would be too much.
Therefore, the best suited choice was to go with an LCD display and interface the display with the MCU using I2C.